The present invention relates generally to MR imaging and, more particularly, to a method and system of reducing artifacts in a phase-cycled steady-state free precession (SSFP) acquisition. Phase-cycled SSFP acquires data multiple times from a given anatomical region, wherein each volumetric acquisition is acquired with a different RF phase increment. Thus, multiple volumes of data of the same anatomical region are acquired. The present invention further relates to implementing a phase-cycled SSFP pulse sequence to acquire MR data for the first volume with a reverse elliptic centric view order and acquire MR data for the second volume with an elliptic centric view order. The present invention also relates to the playing out of a series of dummy acquisitions between imaging of the first and second volumes, and the gradual modulation of RF pulse phase increment while the dummy acquisitions are played out.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Phase-cycled SSFP imaging techniques are commonly used for high resolution imaging of structures such as the internal auditory canal, cervical spine or cartilage imaging, but also can be used for many other applications. SSFP is a technique for generating MR signals wherein hydrogen nuclei do not completely return to their thermal equilibrium state. SSFP pulse sequence imaging relies upon a quasi-steady-state of magnetization in a subject being scanned by applying the SSFP pulse sequence at repetition times (TR) significantly shorter than the spin-lattice (T1) and the spin—spin (T2) relaxation times of hydrogen nuclei in the subject. The SSFP pulse sequence includes a series of RF excitation pulses wherein the pulses typically have the same constant phase increment. SSFP pulse sequences typically achieve high signal-to-noise ratios within short scan times. Images produced by SSFP pulse sequences are also typically very sensitive to motion.
With phase-cycled SSFP, multiple image volumes of the same region of anatomy may be acquired, each with its own steady-state phase-cycling scheme. Cycling the phase (i.e. incrementing the phase by a constant value at each TR) of the RF excitation pulse shifts the spectral frequency response, thus shifting the frequency at which the SSFP signal nulls (or “banding artifacts”) occur. The resulting image volumes are combined to obtain an image of the anatomy with the desired characteristics. For example, to avoid SSFP banding artifacts, the image volumes may be combined using a maximum intensity projection across volumes or Fourier Transform across several volumes. There are also related techniques that use complex addition or subtraction to enhance contrast between water and fat.
To maintain a steady-state during data acquisition, the volumes are generally acquired sequentially. Interleaving data acquisition may be disruptive to the steady-state conditions created in each of the imaging volumes. Because of the sequential acquisitions, there is a high probability of subject motion causing mis-registration of the image volumes. Depending on the method used for combination of the image volumes, mis-registration may blur or obscure structures of interest altogether.
One proposed solution to address and, preferably, reduce motion induced artifacts is predicated upon a reverse centric phase encode acquisition for a first volume followed by a centric phase encode acquisition for a second volume. With a reverse centric phase encode acquisition, k-space is scanned in a spiral pattern such that the periphery of k-space is sampled first during the scan and the sampling continues or spirals inward to acquire the central views of k-space at or near the end of the scan. In contrast, with a centric phase encode acquisition, the center of k-space is sampled early in the scan and sampling continues in a spiraled fashion toward the periphery of k-space. Elliptic centric view order, however, is generally not practical for SSFP pulse sequence protocols.
The SSFP signal typically requires time on the order of three to five times the T1 value of the target tissue to come to a steady-state condition. For cerebrospinal fluid at 1.5T, twelve to twenty seconds is necessary for a steady-state condition to be achieved. Waiting as long as twenty seconds before imaging significantly increases scan time and negatively affects patient throughout. It is also generally well-known that oscillatory behavior is experienced as the signal in the target tissue decays to steady-state. Numerous schemes have been developed to reduce oscillatory behavior, but it has been shown that these schemes work well only for spins near the resonant frequency. Off-resonant spins still exhibit oscillatory behavior. As a result, since elliptic centric ordering acquires the center of k-space temporally first, image quality degradation is possible if the spins have not had sufficient time to decay to steady-state and still exhibit oscillatory signal fluctuations.
It would therefore be desirable to have a system and method capable of elliptic centric phase order acquisition with phase-cycled SSFP pulse sequences for multi-volume imaging that reduces oscillatory conditions as steady-state conditions are achieved.